It's just known that HL do their applications for both cycles (Winter & Summer) on a rolling basis, it can also be found on other websites where the position is posted; they will say they review applications on a rolling basis.How do you know this? I can't find it ANYWHERE on their website
Although it is not very nice of them to not make it obvious on their website, I really wouldn't worry. If you have a strong application, I'm sure it won't matter if it's on a rolling basis.